TUI: kitty keyboard protocol, word movement and deletion keys

Query CSI ? u at startup and push the disambiguate flag on the alternate
screen when the terminal answers; CSI u reports are mapped back to legacy
input (Shift/Ctrl+Enter insert a newline, Ctrl+Backspace deletes a word).
Ctrl/Alt+Left/Right move by word, Ctrl/Alt+Delete delete the next word.
Our define_key codes move above the range ncurses uses for terminfo
extended keys.

docs/compat.md

@@ -87,6 +87,13 @@ take the agent down.
 - Bracketed paste needs ncurses (all platforms above).
 - macOS Terminal.app sends Option+Enter as Esc+Enter only with "Use Option as
   Meta key"; Ctrl+N always inserts a newline.
+- Kitty keyboard protocol: at startup the TUI sends `CSI ? u`; if the terminal
+  answers (kitty, WezTerm, foot, Ghostty, WHC Shell, ...) it pushes the
+  "disambiguate" flag (`CSI > 1 u`) on the alternate screen and pops it on
+  exit. That makes Shift+Enter / Ctrl+Enter (newline), Ctrl+Backspace (delete
+  word) and an instant Esc work. Other terminals (and tmux) don't answer and
+  keep the legacy keys. If the TUI is killed, the main screen's keyboard mode
+  is untouched because the flags live on the alternate screen's stack.
 
 ## Acceptance checklist